Elevated temperature resistant ceramic structural adhesives



March 2, 1965 D. G. BENNETT ETAL 3,171,750

ELEVATED TEMPERATURE RESISTANT CERAMIC STRUCTURAL ADHESIVES Filed Aug. 14. 1961 o R00 600 /ddd JHM @rfa-Nang ,m

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United States Patent O 3,171,750 ELEVATED TEMPERATURE RESISTANT CERAMIC STRUCTURAL ADHESWES Dwight G. Bennett, Champaign, lll., Richard M. Spriggs, Reading, Mass., and Henry G. Lefort, Alamo, Calif., assgnors to the United States of America as represented by the Secretary of the Air Force Filed Aug. 14, 1961, Ser. No. 131,447 8 Claims. (Cl. 10d-43) This invention relates to ceramic adhesives and to a method for their manufacture. In a more specilic aspect, this invention relates to ceramic-oxide glassy-bonded adhesives characterized by a high degree of heat resistance.

i The recent advent of high speed and high altitude aircraft and missiles has created a need for structural bonding materials capable of maintaining their strength and effectiveness at temperatures in the range of 10G0 F. The high ambient temperatures encountered during the operation of high speed propulsion devices, as well as the severe stresses and strains imposed upon their integral components, produces an undesirable weakening eitect in structural elements joined together by previously well-known bonding materials. The use of organic adhesives has proved unsuitable for elevated temperature applications because of their thermal and oxidative instability at temperatures above about 400 F. The refractory adhesives employed heretofore, although highly resistant to oxidative degradation, are limited in use because of their inherent brittleness, low impact resistance, poor thermal shock resistance, and relatively high maturing temperatures.

Accordingly, it is the primary object of this invention to circumvent the above-described limitations of the prior art by providing novel ceramic structural adhesives and a method for their manufacture.

Another object of this invention is to provide novel ceramic adhesives capable of maintaining their shear strength at elevated temperatures in the range of 1GO0 F.

Still another object of this invention is to provide novel ceramic adhesives that are particularly adapted for use in the bonding of structural components subjected to operational conditions of high temperature and severe stress.

A further object of this invention is to provide novel ceramic adhesives that exhibit good thermal shock and impact properties, resistance to moisture and low maturing temperatures.

The above and still further objects, advantages and features of this invention will become readily apparent upon consideration of the following detailed description thereof when taken in conjunction with the accompanying drawings, in which:

FIGURES 1 and 2 are graphical representations disclosing shear strengths of the ceramic adhesives contemplated by this invention.

In accordance with this invention, it has been found that certain refractory ceramic frits of a range of composition to be disclosed in greater detail hereinafter, when combined with colloidal silica and water produce novel ceramic adhesive compositions which are characterized by an unexpected strength at elevated temperatures. These ceramic adhesives are useful in bonding metal-to- 3,l7l,75il Patented Mar. 2, i965 metal, especially stainless steel and'alloys ofthe stainless steel type. Of particular value is the utilization of these adhesives in forming bonded joints in the honeycomb sandwich type of panel construction used for aircraft and missiles.

The ceramic adhesives contemplated b y this invention comprise a ceramic refractory frit, colloidal silica or clay, or some other suitable suspending agent, and water. These ingredients are mixed together and milled to a very line condition called a slip. The adhesive slip can then be directly applied to metallic components by dipping or spraying, allowed to dry, and then tired at an elevated temperature to develop the adhesive bond. In general, the adhesive slip comprises about to ll() parts by Weight of a -ceramic frit, l to 3 parts by Weight of a suitable suspending agent, and about 30 to 60 parts by Weight of water.

The frit component of the ceramic adhesive of this invention Iis made by melting properly selected ceramic oxides, halides or metals into a glass by smelting the desired mixture in a suitable gas-tired smelter for a suicient time and at a temperature until a smooth, pliable thread can be drawn from the melt. It is then quenched in Water which shatters it and makes it easy to grind in the milling operation. The ground mixture is dried and ground to pass through a 4t2- mesh sieve. The frit is then `ready for mixing with the suspending agent and Water. Selection of the frit ingredients is based Yon the physical properties desired in the frit such as viscosity, thermal expansion, and strength. It was found thaththe thermal expansion of the adhesive should approach that Vof the metal in order to obtain highY shear strengths. i' However, it may not be too near to the metal or crazing tendencies may reduce shear strength. A" n In order to illustrate specific embodiments of this invention, there are presented in Tables through 1V detailed examples of the ceramic adhesive slip, as well as the ceramic frit which forms a component part thereof. The ceramic frit composition may be incorporated into the adhesive slip as a unitary component, or a mixture .of frits may be employed.

The composition of the glassy ceramic frits are in the range shown in Table I. Example l thereof discloses a range of composition for one type of frit while Example 2 discloses a different and more simple type of frit.

TABLE I Ceramic frit, parts by weight Exam le Composition p TABLE II Ceramic rt, parts by weight Example Composition Syloid 244 2 Amt.

Frit From- Water Adhesive Example Amt.

Example Amt.

Table II above discloses detailed examples of the ceramic frits contemplated by this invention. The specific chemical ingredients and the amounts thereof come within the range of composition disclosed by Table I.

TABLE III Ceramic frit, parts by weight 1 A1203, dust collector fines.

The raw batch formula for the glassy frit for Examples 3, 4 and 5, respectively, of Table II are presented in Examples 3A, 4A and 5A, respectively, of Table III above. The frits disclosed in Tables II and III are made by melting into a glass a selective mixture of various oxides and halides, then quenching the glass in cold Water to shatter it, followed by a grinding operation. The frit mixture is then ready to be incorporated into the colloidal silica and water in order to produce a ceramic adhesive.

Specific examples of various ceramic adhesives formed in accordance with the teachings of this invention are presented in Table IV below. Examples 6, 7 and 8 comprise a mixture of the glassy frit from Examples 3, 4 and 5 respectively, as well as colloidal silica and water in the amounts indicated. The ceramic adhesive mixture is milled, usually in a porcelain ball mill in a very fine condition called a slip. The iineness of the slip is such that no more than 4 to 6 percent by weight of the residue is retained on a G-mesh sieve. The adhesive slip can then be directly applied to metal pieces to be joined together, allowed to dry, and then ired at a temperature between 1000 F. and 2000 F. to develop an adhesive bond.

1 All batches were milled to a trace on 20G-mesh from a 100 gram sample. 2 Syloid is a colloidal silica slip suspension agent, manufactured by the Davison Chemical Company, Balitmore 3, Maryland.

Evaluation of the ceramic adhesives of this invention is set forth in Tables V through XVI. Evaluation of the adhesives was accomplished by shear testing ceramic adhesive bonded test specimens in the range of room temperature to 1000" F. A Tinius Olsen super L type hydraulic testing machine and a split tube furnace were utilized for this purpose. The split tube furnace was employed to enclose the specimen when it was mounted in the testing machine for the purpose of determining shear strengths at elevated temperatures. The furnace was heated to the test temperature and held at that temperature for l0 minutes in order that it might approach equilibrium. The test specimen was then loaded at 1200 pounds scale reading per minute until failure occurred. The breaking shear stress was doubled to get the shear strength in p.s.i. since the test area of each specimen was only ione-half square inch.

A Rockwell C hardness tester equipped with a diamond penetrator (Brale) and using a major load of kilograms was used to determine the Rockwell C hardness Value of the various specimens evaluated for shear strength. Hardness values were obtained on specimens as received, after they had been subjected to various precipitation hardening treatments, and also after they had been tested in shear at 600 F. and 800 F. The Rockwell C hardness obtained on as received specimens referred to hereinafter as condition A was between 0 and 2.

Types 17-7PH and 302 stainless steel were used to make the test specimens employed in evaluating the ceramic adhesives. The results obtained are based on shear strength values of lap-joined specimens tested at various temperatures in the range of room and 1000 F. The shear test specimens were cut from stainless steel sheet stock *as a thin rectangular section measuring 4%. inches by 1 inch. Each section formed a specimen half and a 1/2 inch end area of each half was cleaned by sandblasting. The sandblasted end of each half was then coated by dipping in the adhesive to be tested, dried, and then the two coated ends were joined together so that a 1/2 inch overlap resulted. The lap-joined specimen, either singly or in a `stack of multiple pieces, was then placed on a stainless steel firing rack. A static cure pressure of 50 p.s.i. was applied to the stack in the direction perpendicular to the individual planes of the overlapped coated ends by means of a suitable dead weight. The specimens were red for about 20 minutes at about l750 F. under the cure pressure so that the glassy adhesive particles in the ceramic adhesive softened and fused together, to the metal, and any screen carrier or similar carrier used di` rectly in the adhesive material, thus forming a continuous glassy matrix phase. The specimens were then allowed to cool in place to room temperature. Specimens of type 302 stainless steel were then ready for shear strength testing but it was necessary to further heat treat type 17-7PH stainless steel to precipitation harden it before shear strength testing. It was unexpectedly found that' the ceramic adhesives of this invention are especially adapted for application during the heat hardening treatment of such steels since the maturing temperature of these adhesives fall within the specifications for the heat hardening treatment for type 17-7PH stainless steel.

Various heat hardening treatments were utilized in precipitation hardening the shear strength test specimens of type 17-7PH stainless steel. The stainless steel was obtained from the supplier in condition A which is mostly austenite, annealed at 1950 F. plus or minus 25 F. and air cooled. rIhe specification for the iii-st type of heat treatment, hereinafter referred to as condition B, cornprises the steps of heating the metal to 1400 F. plus or minus 25 F. for 90 minutes, then cooling it to 60 F. within one hour after removal from the furnace. After cooling the metal was then hardened at 1050 F. plus or minus F. for 90 minutes, followed by air cooling to room temperature. The second type of heat treatment, hereinafter referred to as condition C, was similar in all respects to heat treatment condition B, except that the specimen was hardened at 950 F. plus or minus 10 F. for 30 minutes followed by air cooling to room temperature. rEhe third type of heat hardening treatment, hereinafter referred to as condition D, comprises the steps of heating the metal to 1750 F. plus or minus 15 F. for 20 minutes, cooling to 60 F. within one hour after removal from the furnace, then cooling the metal to 100 F. and holding it at that temperature for eight hours. The metal was then hardened at 950 F. plus or minus 10 F. for one hour, followed by air cooling to room temperature. It is to be particularly noted that the iirst step in the condition D hardening treatment permits the application of the ceramic adhesive completely within the time temperature limits for that treatment.

In most instances, a screen carrier is embedded right in the adhesive bond. The screen becomes an integral part of the adhesive joint and results in considerable improvement in shear strength over that achieved without the use of a carrier screen. A series of regular mesh stainless steel screens of varying mesh iineness and wire diameter, as well as a quartz microfiber paper, were utilized as carriers. A stainless steel carrier screen having a 1,@,2 inch space with a 4 mil wire diameter, Vdesignated as a ZS-mesh space screen, acted to produce higher shear strengths than regular mesh stainless steel screen of a mesh fineness up to 400.

Referring to FIGURES l and 2 there are disclosed the shear strengths achieved by using Various types of screen carriers for bonding types 302 and 17-7PH stainless steel with the ceramic adhesive of Example 6. The stippled bars of FlGURE 1 disclose the shear strength for type 302 stainless steel bonded with the adhesive of Example 6 without the use of a screen carrier. Results achieved by utilizing a screen carrier right in the adhesive bond are also disclosed in FiGURE 1 for both typesl 302 and l7-7PH stainless steel. The specimens of type 302 stainless steel were given the same adhesive firing treatment as the specimens that contained no carrier. It can be seen by an examination of FIGURE 1 that the use of a 28-mesh space screen carrier increased shear strength almost two-fold at all temperatures up to 800 F. The specimens that used a carrier showed the same tendency as the non-carrier specimens of increasing in shear strength as the test temperature increased up to 800 F. A further examination of FIGURE l discloses that 17- VPH stainless steel specimens gave higher shear strengths than type 302 specimens at all temperatures except 800 F. and especially at 900 F. where type 17-7PH stainless steel specimens reached a maximum in shear strength of 1750 p.s.i.

A series of regular mesh stainless steel `screen carriers of varying mesh neness and wire diameter were used with type 302 stainless steel test specimens bonded with the adhesive of Example 6. Shear strength tests were made at room temperature and at 800 F. and these results are presented in FIGURE 2. As is disclosed by an examination of FIGURE 2, the screen carriers acted to 5 generally increase shear strength as the mesh became finer eindl wi'e diameter smaller. Screen carriers of 120 to 400 mesh had about the same effect on shear strength but none of these was as effective as the ZS-mesh stainless steel space screen:

The ceramic adhesive of Example 6 was also subjected tomstr'ess rupture tests. Type 302 stainless steel was bonded with the adhesive and a ZS-mesh' stainless steel space screen wasemployed as a carrier. Stress rupture data was obtained by using Krouse stress rupture racks. The specimens were held at predetermined temperatures and loads and the time required for the specimens to fail was observed. The ceramic adhesive of Example 6 withstood 700 p.s.i.Y for 171 hours at room temperature with n-o failure occurring. At 600 F. this adhesive withstood 600 p.s.i. for- 500 hours Without failure.

TABLE V S h ear strength and Rockwell C hardness R-oclwell CA A Adhesive Shear Hardness N o. Ceramic Temp. of Thickness, Strength, Adhesive Test, F. Mils p.s.i.

- Before' After Test Test Example 0 Room 7 680 35 35 Example 6. Room 8 G60 37 37 Exrn'pl 6 z 600 37 38 Example 6. 600 9 860 37. 38 Example 6. 800 8 S60. 36 37 Example 6 800 8 860 37 38 1 ses' 1 Average. A 2 Specimen broke on removal from furnace due to rough handling.

The shear strength tests for the ceramic adhesive of Example 6, set forth in Table V above,l were determined on shear type specimens of type l7-7FH stainless steel which were hardened to condition B after first being hred at 1750 F. for 2O minutes. Firing the specimens at 1750 F. may have `changed them from the as received condition A; however, they were given the hardening treatment that would ordinarily produce condition B from condition A. The shear strength values for the ceramic adhesives for Example 6 were not greatly increased as a result of the hardening treatment employed. Highs of 680 p.s'.i. at roorn temperature, 860 p.s.i. at '600 F. Vand 800 F. were realized. A 28-mesh stainless steel space screen was employed as a carrier.

VShear strength values for ceramic adhesives of Examples 6 and 7 which had been applied during the condition D heat treatment appeared to be increased due to the hardening treatment. High shear strength values of 1000 p.s.i. at room, 1400 p.s.i.` at 600 F. and 4000 p.s.i. at 800 F. were achieved `with the adhesive of Example 6. The adhesive of Example 7 gave high shear strength values of 960 p.s.i. at room temperature, 1500 p.s.i. at 600 F. and 1800 p.s.i. at 800 F. The Rockwell C hardness values for these specimens Varied between 43 and 46 before shear strength tests and between 44 and 47 after shear strength tests at elevated temperature. These results are set forth in Table VI below.

The adhesives were applied to the sandblasted ends 0f the specimens of Table VI by dipping the ends in each adhesive respectively anddryi-ng at 200 F. The specimens were then fired for 20 minutes at 1750 F. as part of the hardening treatment necessary to reach condition D with a cure pressure of 50 p.s.i. and with the use of a 28-mesh stainless steel space screen carrier. The specimens were then given the remaining hardening treatment to reach condition D" and then tested in shear at room temperature, 600 F. and 800 F.

TABLE Shear strength and Rockwell C hardness of'ceramic adhesives applied on type 17-7PH stainless steel hardened to condition D Rockwell C Adhesive Shear Hardness No. Ceramic Temp. of Thickness, Strength, Adhesive Test, F. Mils p.s.i.

Before After Test Test Example 6 Room 6 1, 000 44 44 Example 6 Room 7 860 45 45 Example 6 600 7 1, 400 45 46 Example 6- 600 7 1, 300 46 46 Example 6 800 7 l, 400 45 47 Example 6 800 7. 4, 000 45 46 Example 7 RoomV 7 96o 4s 4e Example 7. a,..- Room 7 960 45 45 Example 7. 60o 7 1, 50o A4e 45 Example 7. 600 7 1, 500 43 45 Example 7. 800 8 1, 200 46 44 Example 7- 800 7 .1, 800 45 45 l Average. A n

A comparison of Tables V and VI indicates that the ceramic adhesive of Example 6 will give higher shear strength and hardness Values on type 17-7PH stainless steel specimens hardened to condition D than it will on the same steel hardened to condition B. It also indicates that any inversions taking place in the stainless steel during hardening do not adversely afIect the shear strength when using the adhesive of Example 6. The adhesive of Example 7 was also improved strengthwise by hardening the metal to condition D.

TABLE VII Shear strength and Rockwell C hardness of ceramic adhesives applied on type 17-7PH stainless steel hardened to condition D using a 28-mesh stainless steel space screen' carrier Rockwell C Adhesive Shear Hardness No. Ceramic Temp. of Thickness, Strength, Adhesive Test, F. Mils f p.s.i.

Before After Test Test Example 8 Room 6 1, 500 46 46 Example 8 Room 7 1, 360 46 45 Example 8 600 7 l, 400 46 47 Example 8 600 7 1, 540 46 46 Example 8 soo 8 2, 20o 46 47 Example B 800 7 3, 000 46 47 Example s Room 7 1, 39o 4e 4s Example 6 Room 7 1, 250 46 47 Example 6 600 6 1,270 45 47 Example 6 r600 7 1, 210 46 47 Example 6 800 7 2, 150 46 48 Example 6 800 7 1, 270 46 47 l Average.

The ceramic adhesive of Example 8 was designed to have a 10W maturing temperature in the range of 1300 F. to l500 F., a high thermal expansion as close as practical to the expansion of type 17-7PH stainless steel, and to be suciently reactive with the metal to produce a strong adhesive bond. The test specimen ends of type 17-7PH stainless steel were sandblasted, dipped in the adhesive, dried for one hour at 200 F. and fired for 2O minutes at 1750 F. with a cure pressure of 50 p.s.i. using a ZS-mesh stainless steel space screen carrier. Specimens were then given the remaining heat rtreatment required to obtain condition D. Shear strength tests were conducted at room temperature, 600 F. and 800 F. Rockwell C hardness values were also determined before and after shear strength testing. rlfhe results of these tests are set forth in Table VII, above. The adhesive of Example 8 gave relatively good shear strength values as can be seen by comparison of the results set forth in Table VII for the adhesive of Example 6. It is believed that the B203 content extended the maturing range and therefore the specimens were not over-tired during the 20 i minutes of heating at l750 F.

TABLE VIII Rockwell 0" Adhesive Hardness N o. Temp. of Thickness,

Test, F. Mlls Shear Strength, p.s.i.

Ceramic Adhesive After Test Before Test Example 8 l, 000 Example 8 Example 7 Example 7- Room Room Example 7 Example 7 Example 7 Example 7 Example 7- 1, 000 Y Example 7 1, ooo

1 Average.

Table VIII also sets forth shear strength values of the ceramic adhesives from Examples `6 and 8 and includes results obtained at test temperatures of 1000 F. The test specimens of Table VIII were prepared in the same manner as those for Table VII. A ZS-mesh stainless steel space-screen was used with all test specimens and the adhesives were applied during the hardening treatment required to obtain condition D from condition A. The adhesive of Example 6 was prepared from a new adhesive slip in orderto insure the reproducibility of previous test results.

Table IX sets forth shear strength results obtained by using the ceramic adhesive from Example 8. The adhesive was applied to sandblasted shear strength specimens of types l7-7PH stainless steel during the hardening treatment required to reach condition C from condition A. The specimens were dipped in the adhesive, vdried at 200 F. for one hour and then tired at 1400 F.

for 90 minutes with a cure pressure of 50 psi., cooled to room temperature and reheated to 950 F. for 30 minutes. A 22S-mesh stainless steel space screen was used as a carrier. The Rockwell C hardness values were determined before and after shear testing, and shear testing was conducted at room temperature, 600 F. and l000 F. Ceramic adhesives of vExample 8 when applied before the hardening treatment required to reach condition C matured to a glossy bond. It did not, however, Vgive as high a shear strength when applied before the condition C hardening treatment as it did when applied before the condition D hardening treatment. A comparison of Tables 1X and VH give the shear strength values of these specimens given the two diierent heat Vtreatmentsand the results vary in the same manner as the temperature increases. However, the values for the specimens given the condition C hardening treatment are of a lower magnitude. The Rockwell C hardness values averaged about two points less for the condition C hardening treatment than for the condition D hardening treatment. However, after shear strength testing at l000 F. the Rockwell C hardness value dropped by about live points for specimens given either heat treatment. i

TABLE 1X Shear strength tests of a ceramic adhesive applied on type 17-7PH stainless steel and the Rockwell C hardness number for the steel Rockwell C Adhesive Shear Hardness No. Ceramic Temp. of Thickness, Strength, Y Adhesive Test, F. Mils p.s.i.

Before After Test Test Example 8 Room 6 560 43 42 Example 8- Room 6 540 44 44 Example 8 Room 5 540 45 44 Example 8. 600 6 300 45 45 Example 8 600 5 320 44 45 Example 8 600 8 680 44 45 Example 8 2 800 6 44 Example 8. 800 6 770 44 45 Example 8 800 6 1, 060 43 45 Example 8 1, O00 6 80 45 41 Example 8 1, 000 y 5 140 45 39 1 Average. 2 Broken while adjusting specimen grips.

TABLE X Shear strength tests of n ceramic adhesive using a 28meslz stainless space screen as a carrier applied on type 17-7PH stainless steel during hardening to condition D and the Rockwell "C hardness number of the hardened stainless steel specimens Rockwell C Adhesive Shear Hardness No. lCeramic Temp. of Thickness, Strength, Adhesive Tests,v F. Mils psi.

Before After Test Test Example 6 800 6 1,070 45 43 Example 6. 8.00 7 l, 070 43 49 Example 6. 800 6 1, 000 46 46 Example 6. 800 6 l, 006 47 47 Example 6 900 6 1, 836 46 46 Example 6- 900 6 1, 210 46 47 Example 6. 900 6 2, 220 46 47 Example 6 1, 000 7 1,160 46 44 Example 6- 1,000 7 800 45 45 1 Average.

TABLE XI Shear strength tests of a ceramic adhesive using quartz microfiber paper as a carrier on type 17-7PH stainless steel hardened to condition D and the Rockwell "C hardness number of the hardened stainless steel specimens Tables X and XI, above, set forth further shear strength tests of the adhesive from Example 6 using a ZS-mesh stainless steel space screen carrier and a quartz microfiber paper carrier. The specimens were tested in shear at temperatures up to 1000 F. and Rockwell C hardness numbers were determined before and after shear testing. An examination of Table X discloses that shear strength values of 990 p.s.i. at room temperature, 1040 p.s.i. at 600 F., 1035 p.s.i. at 800 F., 1750 p.s.i. at 900 F., and 980 p,s.i. at 1000 F. were obtained on shear specimens of type 17-7PH stainless steel hardened to condition D. Room temperature test results for specimens using a quartz microfiber paper as a carrier approached the shear strength values for the specimens using a 28-mesh stainless steel space screen as a carrier as `is indicated by a comparison of the results set forth in Tables X and XI. However, shear strength for the quartz papier carrier was about p.s.i. lower at 600 F. and 800 F. than for the specimens using the 28-mesh stainless steel space screen as a carrier. Therefore, it is indicated that the use of a metal screen as a carrier will result in higher shear strengths than carriers such as quartz microber paper, especially when using the ceramic adhesives of Example 6.

1 1 TABLE X11 Shear strength tests of a ceramic frz't jlm applied on type 17-7PH stainless steel daring hardening to condition D and the Rockwell "C hardness number of the hardened stainless steel specimen 12 shear specimens bonded with thin films of the ceramic frit from Example 4 and hardened to condition D gave shear strength Values of 670 p.s.i. at room temperature, 300 p.s.i. at 600 F. and 600 p.s.i. at 800 F. An examination of the specimens after shear testing disclosed that R k u C the area of contact between the adhesive and metal was e ,i Adhesive Shear I Hgfdgss No, much less than the half-inch overlap area. Apparently the 51222@ tlpa Thless Stslfh' lm of glass drew up into a smaller area due to surface Before After tension at the maturing temperature of the adhesive. This Test Test indicates that if the thin film of glass could be bonded Examp1e4 Room 1 0 67g 45 44 to the metal over the entire half-inch overlap area, much i122: ggg 33 g g higher shear strengths would result with the use of such very thin lms of adhesives. 1300 1 4 800 l o 84 4 46 Shear strength test results of the ceramic adhesive ,of l 42:2: 800 lo 368 4g 47 Example 6 using line and coarse mesh sintered strands of ,600 stainless steel as carriers to bond type 17-7PH stainless steel are set forth in Table XIII, below. 1 Average.

TABLE XIII Shear strength tests of a ceramic adhesive applied on type 17-7PH stainless steel using sintered stainless steel fiber sheet carriers and the Rockwell C hardness number for the steel Roekwe11C Adhesive Shear Hardness No. Specimen 1 Cerannc Temp. of Thickness, Strength,

o. Adhesive Test, F. Mils p.s.i.

Before .After Test Test 1 Example 6.--- Room 7 890 46 46 2 Example 6-... Room 7 1, 200 47 47 3 Example 6-.-. 600 10 1,080 46 47 4 Exemple 6-.-. 600 11 1,040 46 45 5 Example 6 800 13 3,230 46 45 6 Example 6.--. 800 10 2,870 47 45 7 Example 6.-... 1, 000 12 50 47 39 8 Example 6---- 1, 000 10 60 46 39 Example 6 i Room 8 1,330 46 46 Example 6-... Room 9 1,250 46 45 11 Example 6.-.. 600 7 1,290 46 45 12 Example 6.--. 600 8 1,500 47 46 13 Example 6---- 800 9 2,960 46 45 14 Example 6..-.. 800 8 1,730 47 46 Example6.- 1,000 8 430 46 30 Example 6---. 1,000 9 360 45 3s 1 Specimens 1 through 8 used stainless steel fiber sheets with f ne voids and specimens 9 through 16 used stainless steel ber sheets with larger voids as a carrier.

2 Average.

Table XII, above, sets forth the results obtained when an attempt was made to fabricate a very thin layer of glass that could be used as an adhesive. The adhesive frit of Example 4 was resmelted in a small crucible. A ceramic tube was inserted in the melt and a molten gob of glass was collected on its end. This gob was then blown into bubbles of very thin wall thickness. The resulting films of glass were used as adhesives in sandblasted lap joint shear specimens which were red for 20 minutes at 1750 F. with a cure pressure of p.s.i. The specimens were then given the remaining hardening treatment necessary to reach condition D. Test results are set forth in Table less steel space screen carrier as can be determined by comparing with the results of Table X. The only significant increase in shear strength, however, was at 800 F. when using the stainless steel sintered carrier where 3050 p.s.i. was recorded compared to 1035 p.s.i. when XII. An examination of Table XII discloses that the using a 28-mesh space screen carrier. The coarser mesh sintered stainless steel carrier gave a few hundred p.s.i. lower value at room temperature and at 600 F. than the liner mesh sintered carrier, but gave 700 p.s.i. lower at 800 F. than the line mesh sintered carrier. In general, this would indicate that a sintered stainless steel carrier is as effective in increasing shear strength as is the 28- mesh stainless steel space screen when using an adhesive of the type set forth in Example 6.

in shear at room temperature. All but one of the adhesives subjected to 100 hours immersion in boiling water decreased in shear strength. The adhesive of Example 8, which -is `made of a somewhat soluble adhesive frit was dissolved during the boiling water test to an extent as to completely lose its adhesive qualities. The adhesive of Example 6 decreased about 25 percent in shear strength due to the boiling water test.

lTABLE VXV Elevated temperature stress rupture data :for ceramic adhesive of Example 6 applied on type 17-7PH szanless steel 1 All specimens used a 28-mesh stainless steel space screen as a carrier.

2 The ceramic adhesive was applied during the hardening treatment required to obtain condition D from condition A.

3 Two specimens broke duringapplication of a 1,000 p.s.i. load.

4 Specimens 2 and 3 were pulled in sheer at 800 F. alter they had completed 1,000 hours in stress-rupture.

TABLE XIV .Shear strength tests of ceramic Vadhesives applied on type .I 7-7PH stainless steel after nfzmersoriv in boiling water for 100 hours and the Rockwell C hardness number for the steel Rockwell C Immersed Adhesive Shear Hardness No. Ceramic and Thickness, Strength, Adhesive Control Mils p.s,i.

Specimens Before After Test Test Example 8 1 4 46 46 Example 8. 8 46 4G Example 8- 3 1, 210 46 4G E xample 8. 3 1, 320 46 46 Example 6 Imm 3 980 46 44 Example G Imm 2 930 44 44 Example G Cont 4 1, 330 45 44 Example (i- Cont 3 1, 300 45 44 1 All specimens were tested in shear at room temperature. Specimens bonded With the adhesive of Example 8 separated during immersion in boiling water.

2 Average.

Table XIV, above, sets forth shear strength tests for the ceramic adhesives of Examples 6 and 8 when applied on type l7-7PH stainless steel without the use of a carrier after immersion in boiling Water for 100 hours. These specimens were hardened to condition D. The specimens bonded with each of the adhesives were submerged in boiling tap water at 212 F. for 100 hours. These specimens, as well as two specimens bonded with each adhesive but not subjected to boiling water, were tested Stress rupture tests for 'the ceramic adhesive of Example 6 are set forth in Table XV, above. Shear test specimens of type 17-7PH stainless steel were bonded with the adhesive and then subjected to the stress rupture tests. Stress rupture data were accumulated at 800 F. and 900 f F. by means of Krouse stress rupture racks. The load was applied to the specimens, mounted in the stress rupture rack, at a uniform rate up to the desired shear level, by means of a hydraulic jack which supported the load before its application to the shear specimen. In the stress rupture test, the ceramic adhesive of Example 6 when used to bond 17-7PH stainless steel hardened to condition D," withstood a 700 p.s.i. static load for 136 hours at 900 F. before failure. Another test specimen was retired after completing 1000 hours under a load of 700 psi, at 800 F. It was then tested in shear at 800 F. and developed 1470 p.s.i. before failure. Since this value is representative of this adhesive at 800 F., it is indicated that the static load of 700 p.s.i. at 800 F. did not affect the structure of the adhesive bond. A third specimen completed 1000 hours under a load of 800 p.s.i. at 800 F. without failure. This specimen later developed 1900 p.s.i. in shear at 800 F. before failure. All of the test specimens when subjected to stress rupture tests broke when a load of 1000 p.s.i. was applied at room temperature. Even though the load was applied to the specimen very slowly, by means of the hydraulic jack, failure still occurred. It can be noted from an examination of FIG- URE 1 that specimens of type 17-7PH stainless steel prepared in a similar manner using the same constituents had an average breaking shear` strength of 1060 p.s.iy at room temperature. Failure during the stress rupture testing may have been due to attempting to apply a load (1000 psi.) which was too near to the average breaking shear strength values for these specimens.

1 5 TABLE XVI Shear strength tests of a ceramic adhesive after long heating and thermal shock cycling Rockwell C Adhesive Shear Hardness No. Ceramic Temp. of Thickness, Strength, Adhesive Test, F. Mils p.s.i.

Before After Test Test Example 6 Room 4 1, 400 34 34 Example 6 Room 4 1, 510 34 34 Example 6- 600 5 1, 250 35 3G Example 6 600 5 (2) Example 6------ 800 5 1, 400 34 35 Example 6- 800 5 2, 460 35 35 Example 6. 1,000 4 1, 240 34 34 Example 6 1, 000 3 1, 230 34 34 1 Average 2 Broke before long heating and thermal shock cycling.

Table XVI, above, sets forth shear strength tests of the Ceramic adhesive of Example 6 which had been subjected to a long heating and thermal shock cycling.

Shear specimens of 17-7PH stainless steel were bonded with this adhesive in the usual manner except that no screen carrier was used. The shear specimens were then subjected to a long heating and thermal shock cycling using a furnace at 1000 F. The test specimens 35 were hardened to condition D. All the specimens were subjected to 143 hours of heating at 1000 F. and thermal shock cycled 25 times between 1000 F. and room temperature before being tested in shear. The

test specimens yielded excellent shear strengths that 4.0

can be seen by an examination of Table XVI. The adhesive of Example 6 varied in averageshear strength between 1235 p.s.li. and 1930 p.s.i. over the temperature range of room to 1000 F. These values seem improved over the values previously obtained for these adhesives when they were applied in conjunction with a screen carrier, but not subjected to long heating and thermal shock cycling. This seems to indicate that the ceramic adhesives of this 4invention are not harmed by long heating at 1000 F. and thermal shock cycling 50 from 1000 F. In fact, such treatment seems to actually show some benefit for increasing shear strength. Since none of the specimens subjected to long heating and thermal shock cycling used a screen carrier, it is further indicated that a carrier is not always essential in addition, are capable of maintaining their resistance to high temperatures for extended periods of time. The ceramic adhesives -of this invention can be successfully employed for bonding stainless steel and stainless steel alloys, and in particular, can be applied during the precipitation hardening treatment necessary to harden type 17-7PH stainless steel, The unexpected increase in :shear strength for bonded joints employing the adhesives `of this invention has proved to be of special value when 'the adhesive is employed in bonding structural components that are subjected to the stress and elevated temperature conditions encountered during the operation .of high performance jet and rocket aircraft.

Although the present invention has been described `with particular reference to specific embodiments theretof, the vinverltion j s not to be considered as limited thereto, but includes within its scope such modifications and alterations as come within the appended claims.

What is claimed is:

l. A ceramic adhesive composition consisting essentially of a mixture of the following constituents in approximately the -following parts by weight:

Ceramic frit -110 Suspending agent 1-3 Water 30-60 in which said ceramic frit consists essentially of the following materials in approximately the following parts by weight:

Sio2 23-28 A1203 10-15 Y B203 3-6 Nazo 10-20 x20 3 6 BaO 4-7 ZnO 8-12 CaO 4-6 IqagSlFe 4 6 P205 1 5 C1`203 0-5 F6203 0--5 2. A ceramic adhesive as defined in claim l in which said ceramic frit consists essentially of the folowing materials in approximately the following parts by weight:

s-io2 27.2 Nazo 16.3 B203 4.0 A1203 13.0

2O 5.1 BaO 6.0 CaO 5.4 ZnO 1l. NaZSiF 5.0 P205 4.0 V205 3.0

3. A ceramic adhesive as defined in claim l in which said cera-mic frit consists essentially of the following materials inv approximately the following parts by weight:

4. A ceramic adhesive composition consist-ing essentially of a mixture of the following constituents in approximately the following parts by weight:

Ceramic frit 90-110 Suspending agent 1-3 Water 30-60 lin which said ceramic frit consists essentially of the following materials in approximately the following parts by weight:

sio2 37-43 13203 50-60 Nago 3 7 17 5. A ceramic adhesive as defined in claim 4 in which said ceramic frit consists essentially of the following inaterials in approximately the following parts by weight:

SOZ 38.0 N320 B203 57.0

6. A ceramic adhesive composition consisting essentially of a mixture of the following constituents in approximately the following parts by weight:

Ceramic frit 100 Colloidal silica 2 Water 50 in which said ceramic frit consists essentially of the following materials in approximately the lfollowing parts by weight:

SiOz 27.2 N320 B203 4.0 A1203 13.0 KO 5.1 BaO 6.0 CaO 5.4 ZnO 11.0 Na2SF6 5.() P205 4.0 V205 3.0

7. A ceramic adhesive composition consisting essentially of a mixture of the following constituents in approximately the following parts by weight:

Ceramic frit 100 Colloidal silica 2 Water 50 in which said ceramic frit consists essentially of the following materials in approximately the following parts by weight:

8. A ceramic adhesive composition consisting essentially of a mixture of the following constituents in approximately the following parts by weight:

Ceramic frit 100 Colloidal silica 2 Water 40 in which said ceramic frit consists essentially of the following materials in aproximately the following parts by Weight:

Si02 38.() Nago 5.0 B203 57.0

References Cited bythe Examiner UNITED STATES PATENTS 2,250,457 7/41 Bahnsen et al. 106--48 2,706,692 4/55 Chester 106-48 3,114,646 12/63 Currie 10G-48 TOBIAS E. LEVOW, Primary Examiner. JOSEPH REBOLD, Examiner. 

1. A CERAMIC ADHESIVE COMPOSITION CONSISTING ESSENTIALLY OF A MIXTURE OF THE FOLLOWING CONSTITUENTS IN APPROXIMATELY THE FOLLOWING PARTS BY WEIGHT: 